Note 1: The conditions determining operability and committability must be specified.
Note 2: Expressed mathematically, unavailability is 1 minus the availability.
Note 3: Unavailability may also be expressed mathematically as the ratio of the total time a functional unit is not capable of being used during a given interval to the length of the interval, e.g., if the unit is not capable of being used for 68 hours a week, the unavailability is 68/168.
Source: from Federal Standard 1037C and from MIL-STD-188